Corrected RFC number for WebDAV in PROPFIND docs

The comments in Client.propfind() refer to RFC 4916 which is
"Connected Identity in the Session Initiation Protocol (SIP)"
https://tools.ietf.org/html/rfc4916

I guess the right one is RFC 4918 as in the other API docs:
"HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)"
https://tools.ietf.org/html/rfc4918
Merge pull request #8 from Sentido-Labs/docs-typo-fix

Corrected RFC number for WebDAV in PROPFIND docs